PASADENA, Calif. — The verbal battle in the late-night wars got louder Thursday as a top NBC executive struck back against on-air attacks on the network’s once and future “Tonight Show” host Jay Leno.

The executive, Dick Ebersol, chairman of NBC Universal Sports, said the reason for Mr. Leno’s return to NBC’s late-night roster after a short stint in prime time this season was a simple one: disappointing ratings for Conan O’Brien’s “Tonight Show.”

Referring to the pointed jokes made this week by Mr. O’Brien and David Letterman of CBS, Mr. Ebersol said it was “chicken-hearted and gutless to blame a guy you couldn’t beat in the ratings.”

He added that “what this is really all about is an astounding failure by Conan.” Mr. Ebersol is a veteran at the network, with a longstanding relationship with NBC Universal’s embattled chief executive, Jeff Zucker. Mr. Ebersol also has a deep link to the network’s late-night history, having been a creator of “Saturday Night Live,” and he has been frequently consulted on changes in NBC’s late-night lineup.

Ratings for Mr. O’Brien’s “Tonight Show” have dropped off sharply from what they had been under Mr. Leno. Mr. Leno himself experienced a prominent failure when he moved to prime time with a show in September that struggled so much that NBC’s affiliated stations demanded change.

But the change NBC decided on was to reinstate Mr. Leno at 11:35 each weeknight and nudge Mr. O’Brien back a half hour — an outcome he expressly rejected in a statement he issued earlier this week. (Since then, the two sides have been in intense negotiations to settle Mr. O’Brien’s contract. While no deal was expected Thursday night, one executive who has been connected to the talks said an agreement by Friday was possible.)

Separately, NBC announced on Thursday changes to its 10 p.m. hour to take effect after the Winter Olympics. It will introduce a new comedy reality show from Jerry Seinfeld called “The Marriage Ref” on Thursdays and add a new drama, “Parenthood,” as the 10 p.m. show on Tuesdays.

NBC will also move “Law & Order” to Mondays at 10, starting March 1. A sibling series, “Law & Order: SVU,” will go back to 10 p.m., where it once thrived, now on Wednesdays. The network will keep its “Dateline NBC” newsmagazine on Friday at 10 p.m.

Mr. Ebersol chided Mr. O’Brien for declining to take advice about how to adjust his show to the 11:35 p.m. slot from the style he had used on NBC’s 12:35 a.m. “Late Night” show for 16 years.

He said he had met personally with the host three weeks before he stepped behind the “Tonight” desk for the first time to urge him to take steps to expand the appeal he had built up in his “Late Night” years, saying that NBC hosts beginning with Johnny Carson had recognized the importance of making the show appealing first and foremost to cities in the central time zone like Chicago and Des Moines.

Mr. O’Brien’s camp, while steering clear of commenting on Mr. Ebersol’s criticism, confirmed the executive had met with the star and discussed potential changes in the show.

They have previously defended the performance of the show, saying seven months was not a fair shot for Mr. O’Brien to hone his comedic voice at the earlier hour especially in the face of reduced audiences for Mr. Leno’s 10 p.m. show and the late local newscasts that followed it.

Mr. Ebersol labeled that a “specious argument,” saying that for much of the last five years, Mr. Leno had much lower lead-in audiences than Mr. Letterman got at CBS and yet he always won in the ratings.

Mr. Ebersol’s comments came after increasing assaults on Mr. Zucker and Mr. Leno across the late-night shows. Mr. O’Brien on Wednesday began turning to more pointed jokes about Mr. Leno in his monologue, though the most ferocious attacks came from a seeming bystander, Mr. Letterman.

Mr. O’Brien joked that young people should be inspired to believe that they can “do anything you want in life — unless Jay Leno wants to do it, too.”

Mr. Leno had his own fun during his monologue on Thursday, saying: “With all the controversy going on here at NBC, actually, ‘The Tonight Show’ with Conan O’Brien’s ratings have gone up. So you’re welcome.”

Mr. Ebersol said Mr. Leno had not pushed for any of the changes, not the original decision to guarantee Mr. O’Brien the show five years in advance, nor the plan to put Mr. Leno in prime time.

“Jeff and I are big boys,” Mr. Ebersol said, referring to Mr. Zucker. “When we do something big in the public forum and it doesn’t succeed, we know we’ll be the butt of criticism. But you don’t personally attack someone who hasn’t done anything.” In this case, he added, “we bet on the wrong guy.”

Mr. Leno dominated the late-night ratings since the mid-1990s, rarely losing even a night to Mr. Letterman. But NBC faced a dilemma in 2004 when Mr. O’Brien, at the time one of the hottest stars in television, had offers to jump to a different network. To prevent that — because Mr. Zucker and others concluded Mr. O’Brien represented NBC’s future in late night — Mr. O’Brien was offered “The Tonight Show” after a five-year wait.

The recent proposed shift to a later time slot has resulted in a surge of sympathy for Mr. O’Brien, stirring what is playing out as a growing youth revolt among the late-night audiences.

Mr. O’Brien’s fans among the younger segments of the late-night audience have rallied to his defense, forming support groups on the Internet and planning rallies outside his studio at Universal City in Los Angeles. And his ratings seem to be growing.

While overnight household ratings showed Mr. Letterman still ahead on Wednesday, Mr. O’Brien is seeing his best numbers there in months. Mr. Letterman had a 3.5 rating Wednesday night in those preliminary numbers, while Mr. O’Brien grew to a 3.0, well up from a recent average of about a 2.2.

More telling were early demographic numbers from the country’s top 24 cities: there, among viewers ages 18 to 49 — the central age group for most late-night advertisers — Mr. O’Brien seems to be thriving. He climbed to a 1.8 in that group Wednesday, well above the 1.0 he had recently been scoring.

But if Mr. O’Brien was soaring, Mr. Letterman was roaring Wednesday night, unleashing a torrent of biting commentary about NBC, and digging into his apparent lingering bitterness about how it threw him over for Mr. Leno in the early 1990s.

In his monologue, Mr. Letterman dealt slams on Mr. Leno’s grabbing for every host job imaginable, including one joke that had Mr. Leno climbing out of Merv Griffin’s grave. Mr. Letterman also made several references to an incident from that period when Mr. Leno had secretly listened in on an NBC executive meeting from a closet.

“They’re just striking out at Jay,” Mr. Ebersol said. “It seems like professional jealousy.”

A version of this article appears in print on January 15, 2010, on page B1 of the New York edition with the headline: Executive Leaps to Leno’s Defense. Order Reprints|Today's Paper|Subscribe